Enclosures for high power wireless power transfer systems

ABSTRACT

The disclosure features wireless power devices for receiving power from a wireless power source. The devices include a first plurality of magnetic material pieces of substantially planar shape arranged in a first plane, where the first plurality of magnetic material pieces have a first planar surface and a second planar surface. The devices include a device resonator comprising at least one wound conductor disposed on the first planar surface and a second plurality of magnetic material pieces in a second plane, where at least one of the second plurality of magnetic material pieces overlaps at least one of the first plurality of magnetic material pieces and where a separation between the first and second planes is less than 2 mm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/143,345, filed on Apr. 6, 2015, and is a continuation-in-part ofU.S. patent application Ser. No. 14/794,714 filed on Jul. 8, 2015, andpublished as US 2016/0012967A1 on Jan. 14, 2016, the entire contents ofeach of which are incorporated herein by reference.

U.S. patent application Ser. No. 14/794,714 claims priority to U.S.Provisional Patent Application No. 62/022,133, filed on Jul. 8, 2014,and to U.S. Provisional Patent Application No. 62/051,647, filed on Sep.17, 2014, the entire contents of each of which are incorporated hereinby reference.

TECHNICAL FIELD

This disclosure relates to the field of wireless power transfer systemsand methods.

BACKGROUND

Energy can be transferred from a power source to a receiving deviceusing a variety of known techniques such as radiative (far-field)techniques. For example, radiative techniques using low-directionalityantennas can transfer a small portion of the supplied radiated power,namely, that portion in the direction of, and overlapping with, thereceiving device used for pick up. In this example, most of the energyis radiated away in directions other than the direction of the receivingdevice, and typically the transferred energy is insufficient to power orcharge the receiving device. In another example of radiative techniques,directional antennas are used to confine and preferentially direct theradiated energy towards the receiving device. In this case, anuninterruptible line-of-sight and potentially complicated tracking andsteering mechanisms are used.

Another approach is to use non-radiative (near-field) techniques. Forexample, techniques known as traditional induction schemes do not(intentionally) radiate power, but use an oscillating current passingthrough a primary coil, to generate an oscillating magnetic near-fieldthat induces currents in a near-by receiving or secondary coil.Traditional induction schemes can transfer modest to large amounts ofpower over very short distances. In these schemes, the offset tolerancesbetween the power source and the receiving device are very small.Electric transformers and proximity chargers use these traditionalinduction schemes.

SUMMARY

In a first aspect, the disclosure features wireless power devices forreceiving power from a wireless power source. The wireless power devicescan include a first plurality of magnetic material pieces ofsubstantially planar shape arranged in a first plane, where the firstplurality of magnetic material pieces have a first planar surface and asecond planar surface. The devices can include a device resonatorcomprising at least one wound conductor disposed on the first planarsurface and a second plurality of magnetic material pieces in a secondplane, where at least one of the second plurality of magnetic materialpieces overlaps at least one of the first plurality of magnetic materialpieces and where a separation between the first and second planes isless than 2 mm.

Embodiments of the wireless power devices can include any one or more ofthe following features.

The separation between the first and second planes can be less than 0.5mm. The coupling k between the device resonator and a source resonatorof the wireless power source can be at least 5% greater compared to adevice resonator without the second plurality of magnetic materialpieces. The coupling k between the device resonator and a sourceresonator of the wireless power source can be at least 10% greatercompared to a device resonator without the second plurality of magneticmaterial pieces. The separation between the first and second planes canbe uniform. The separation can be maintained using a plastic spacerbetween the first plurality and the second plurality of magneticmaterial pieces.

The overlap can be approximately equal to or greater than 8 mm. Theoverlap can be on the order of a thickness of the first plurality ofmagnetic material pieces or the second plurality of magnetic materialpieces. The first plurality of magnetic material pieces can each have athickness between and including 5 mm to 8 mm. The second plurality ofmagnetic material pieces can each have a thickness between and including2 mm to 5 mm.

The coupling k between the device resonator and a source resonator ofthe wireless power source can be at least 5% greater compared a deviceresonator without the second plurality of magnetic material pieces. Thecoupling k between the device resonator and a source resonator of thewireless power source can be at least 10% greater compared a deviceresonator without the second plurality of magnetic material pieces.

The gap in the second plurality of pieces near a center of the at leastone wound conductor can be approximately 0.5 mm or greater. The at leastone conductor can be wound in the second plane.

The wireless power devices can further include a capacitor network thatis coupled to the at least one wound conductor and positioned under thesecond plurality of magnetic material pieces. The wireless power devicescan further include a conductor piece positioned between the secondplurality of magnetic material pieces and the capacitor network.

The wireless power devices can further include an aluminum shieldpositioned adjacent to the first plurality of magnetic material piecesopposite the at least one conductor. The aluminum shield can be attachedto an underside of a vehicle.

Embodiments of the systems, methods, and coils can also include any ofthe other features disclosed herein, including features disclosed inconnection with different embodiments, in any combination asappropriate.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this disclosure belongs. In ease of conflict withpublications, patent applications, patents, and other referencesmentioned or incorporated herein by reference, the present disclosure,including definitions, will control. Any of the features described abovemay be used, alone or in combination, without departing from the scopeof this disclosure. Other features, objects, and advantages of thesystems and methods disclosed herein will be apparent from the followingdetailed description and figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an embodiment of a wireless powertransfer system.

FIGS. 2A and 2B are schematic diagrams of devices configured towirelessly receiver power.

FIG. 3 is a schematic diagram of a receiver resonator.

FIG. 4 shows a cross-sectional view of an exemplary embodiment of awireless power transfer system.

FIGS. 5A-5B show models of an exemplary embodiments of magnetic materialconfigurations for a wireless power device.

FIGS. 6A-6C show top views of exemplary embodiments of back plates forwireless power transfer systems.

FIG. 7A shows a bottom view of an exemplary embodiment of components fora wireless power source. FIG. 7B shows a side view of an exemplaryembodiment of components for a wireless power source.

FIG. 8A shows a bottom view of an exemplary embodiment of components fora wireless power source. FIG. 8B shows a side view of an exemplaryembodiment of components for a wireless power source.

DETAILED DESCRIPTION

Highly resonant wireless power transfer systems may comprise highquality factor resonators that may be driven to generate oscillatingelectromagnetic fields and that may interact with oscillating magneticfields to generate currents and/or voltages in electronic circuits. Thatis, energy may be transferred wirelessly using oscillating magneticfields via coupled magnetic resonators. In exemplary embodiments, highlyresonant wireless power transfer systems may be integrated into vehiclecharging systems. For example, an electric or electric-hybrid vehiclebattery may be charged via wireless power transfer as described here. Inother exemplary embodiments, a wireless power source may be used totransfer power to a wireless power device which ultimately can deliverpower to electronics in or on a vehicle. In embodiments, a wirelesspower transfer system may provide power greater than 1 kW, 3 kW, 5 kW,10 kW, 20 kW, 50 kW, or more.

In vehicle applications, resonator enclosures may be necessary for thesuccess of wireless energy transfer as well as the protection of theenclosed components. Resonator enclosures may be designed for mechanicalstability and thermal regulation of the components such as one or moreresonators, electronics, magnetic materials, etc. These designconsiderations may be balanced by requirements of the enclosure to be acertain size, shape, or weight.

The components of a wireless power device may be positioned and shapedsuch that the wireless power device packaging is compact and thewireless power device can efficiently receive power from a wirelesspower source and provide power to a battery (such as that of a vehicle).For example, specific positions of magnetic material relative to theconductor of the device resonator can increase the capture of magneticflux from the source resonator and thus achieve greater systemefficiency. The positions of the magnetic material may further simplifythe complexities involved in machining and manufacturability of theoverall resonator structure and enclosure.

FIG. 1 shows a schematic diagram of an embodiment of a wireless powertransfer system 100 that includes a wireless power source 101 and device107. Wireless power source 101 includes a source resonator 102 coupledto source electronics 104, which are connected to a power supply 106.Source electronics 104 can include a variety of components including anAC/DC converter, an amplifier, and an impedance matching network. Powersupply 106 can include one or more of AC mains, solar panels, and one ormore batteries. Not all of the components of power source 101 need to bepresent for operation, and in some embodiments, certain components shownin FIG. 1 can be integrated (source electronics 104 and power supply 106can be integrated into a single component, for example).

Device 107 includes a device resonator 108 coupled to device electronics110 to provide power to a load 112. Device electronics 110 can include avariety of components, such as a rectifier and/or an impedance matchingnetwork. Load 112 generally corresponds to any of a variety ofpower-dissipating electrical components, such as a battery and/or anelectromechanical device. Not all of the components of device 107 needto be present for operation, and in some embodiments, certain componentsshown in FIG. 1 can be integrated (device electronics 110 and load 112can be integrated into a single component, for example).

Source electronics 104 and device electronics 110 can each include oneor more electronic processors (processors 105 and 111, respectively).Electronic processors 105 and 111 can perform a variety of monitoring,computation, and control functions.

Additional aspects and features of wireless power transfer systems aredisclosed, for example, in the following, the entire contents of each ofwhich are incorporated herein by reference: U.S. Patent ApplicationPublication No. 2012/0119569; U.S. Patent Application Publication No.2015/0051750; U.S. Pat. No. 8,772,973; U.S. Patent ApplicationPublication No. 2010/0277121; and U.S. Pat. No. 8,598,743.

In some embodiments, a device configured to receive power wirelessly canhouse both a device resonator and device electronics an integratedmanner. FIG. 2A shows, on the left side, a schematic diagram of anembodiment of a device configured to wirelessly receive power in which adevice resonator coil 202, magnetic material 204, and a conductiveshield 206, such as aluminum, are stacked onto one another. Forreference, a coordinate axis 250 is also shown in conjunction with theschematic diagram. The device in FIG. 2A extends in the horizontalx-direction (and also the y-direction), and has a thickness that ismeasured in the vertical z-direction. FIG. 2A also shows, on the rightside, a perspective view of the device, with coordinate axis 252 shownfor reference. Coordinate axes 250 and 252 are oriented in the samedirections with respect to the features of the device. FIG. 2B shows, onthe left side, a schematic diagram of another embodiment of a deviceconfigured to wirelessly receive power. The device of FIG. 2B has a“top-hat” configuration, in which a center portion of magnetic material208 is stepped in the Z-direction to define an empty region betweenmagnetic material 208 and shield 206. Some or all device electronics 210can be positioned within the empty region and coil 202 is wound aroundthe stepped edges of magnetic material 208. By enclosing deviceelectronics 210 within the device resonator as shown in FIG. 2B, thecompactness of the device can be significantly increased. FIG. 2B alsoshows, on the right side, a perspective view of the device. Coordinateaxes 250 and 252 are oriented in the same directions with respect to thefeatures of the device in FIG. 2B as in FIG. 2A.

FIG. 3 shows a schematic diagram of a receiver resonator that includes aresonator coil 302, a magnetic member 304, a first conductive shield306, and a second conductive shield 308. The receiver resonator ispositioned in proximity to a vehicle chassis 310 formed of steel. Secondconductive shield 308 is formed of aluminum, and is square in shape witha side length 312.

FIG. 4 shows a cross-sectional view of an exemplary embodiment of awireless power transfer system. The wireless power transfer systemincludes a wireless power source 402 and a wireless power device 404.The wireless power source 402 can include the source resonator,including conductor 406, and the source electronics 408. The one or morecapacitors or capacitor networks of the source resonator may be includedin the space for the source electronics 408. In embodiments, some or allof the source electronics 408 may be in the source packaging. It may beadvantageous to have all of the source components within the packagingof the source so that wires carrying large currents, especially ACcurrent, can be shortened. This may be helpful to reduce emissions andlosses from those wires. The source 402 may draw power 409 from anexternal power supply such as AC mains, a battery, solar cell, and thelike. The source 402 may communicate with an external controller, suchas a server or user's computer, and/or the device 404 via one or morecommunication channels 411. In embodiments, the source 402 may be placeddirectly on the ground 403 or surface over which a vehicle 405 equippedwith a wireless power device 404 drives over. In embodiments, the source402 may be embedded into the ground 403 such that the top of the source402 is flush with the surface of the ground 403 or buried into theground 403 such that the source 402 is completely under the surface ofthe ground 403. The source resonator and source electronics may bepackaged such that the expected performance of wireless energy transferis minimally or not affected. The components of the source 402 may bepackaged such that the environment has little or no effect on theperformance of wireless energy transfer.

The source 402 in FIG. 4 includes magnetic material 410 in a first plane(“peripheral” magnetic material) under the conductor 406 of the sourceresonator and over a support structure 412. The support structure 412elevates the magnetic material 410 and source resonator coil 406 awayfrom the back plate 414 and towards the device 404. In some embodiments,the support structure 412 may be made of a conductive material. In otherembodiments, the support structure 412 may be made of a non-conductivematerial such as a plastic. The back plate 414, whose electromagneticcharacteristics can vary, represents a known loss to the system comparedto the ground. In embodiments, the thickness of the back plate 414 canbe determined by the minimum mechanical rigidity needed for the source.For example, the back plate 414 can be made of aluminum with a thicknessequal to or greater than 1 mm. In some embodiments, the aluminum platethickness can be determined by skin depth related to the operatingfrequency (80-90 kHz) of the wireless power transfer system. Forexample, a sufficient thickness of aluminum can be approximately equalto greater than 0.3 mm at 85 kHz. The source 402 in FIG. 4 also includesa magnetic material 416 in a second plane (“top-hat” magnetic material)that guides flux away from any electronics 408 positioned underneath andforces magnetic flux in the center of the source resonator to be higher,which may allow for greater coupling (“boost” in coupling) between thesource and device resonators. The positions of the magnetic materialpieces relative to the other components can be reinforced by spacers,such as plastic, plastic molds, or by thermal interface material. Inembodiments, the source electronics may be further shielded using aconductive material 413, such as copper, positioned between the sourceelectronics and the magnetic material. The source electronics 408 can bepositioned in plane or partially in plane with magnetic material 410and/or the conductor 406. In embodiments, the outer packaging of thesource resonator can include a back plate or shield 414 and cover 418.

The wireless power device 404 can include the device resonator,including conductor 420, and the device electronics 422. The one or morecapacitors or capacitor networks of the device resonator may be includedin the space for the device electronics 422. In embodiments, some or allof the device electronics 422 may be included in the device packaging.The device 404 may provide captured power 423 to the battery, batterymanager, and/or to another component of the vehicle to which it isaffixed. The device 404 may communicate with an external controller,such as the vehicle 405 or a user's computer, and/or the source 402 viaone or more communication channels 425. The device 404 may be positionedon the underside or other surface of a vehicle 405. The device resonatorand device electronics may be packaged such that the expectedperformance of wireless energy transfer is minimally or not affected.The device resonator and device electronics may be packaged to minimizethe overall weight of the device 404. The device components may bepackaged to minimize the effect of the environment, such as the lossymaterial of the vehicle 405. The device may have to also withstandtravel and exposure to different environments that the underside of avehicle may experience.

The device 402 in FIG. 4 includes magnetic material 424 in a first plane(“base” magnetic material) and magnetic material 426 in a second plane(“peripheral” magnetic material). The positions of the magnetic materialpieces relative to the other components can be reinforced by spacers,such as plastic, plastic molds, or by thermal interface material. Inembodiments, the device electronics may be further shielded using aconductive material 427, such as copper, positioned between the deviceelectronics and the magnetic material. The device electronics 422 can bepositioned in plane or partially in plane with magnetic material 426and/or the conductor 420. In embodiments, the outer packaging of thesource resonator includes a back plate 428 and cover 430. Inembodiments, an additional piece of metal 432, such as aluminum, can beused between the back plate 428 and the vehicle. This additional piece432 may be conformal to the underside of the vehicle. In embodiments,the additional piece of metal 432 may be electrically isolated from theback plate 428, for example, by using non-conductive materials. Inembodiments, the additional piece of metal 432 may be thermally coupledto the back plate 428 to allow heat from the device components to bedissipated into the additional piece of metal 432. The back plate 428,whose electromagnetic characteristics can vary, represents a known lossto the system compared to the vehicle. In embodiments, the thickness ofthe back plate 428 can be determined by the minimum mechanical rigidityneeded for the source. For example, the back plate 428 can be made ofaluminum with a thickness equal to or greater than 1 mm. In someembodiments, the aluminum plate thickness can be determined by skindepth related to the operating frequency (80-90 kHz) of the wirelesspower transfer system. For example, a sufficient thickness of aluminumcan be approximately equal to greater than 0.3 mm at 85 kHz.

FIGS. 5A-5B show models of an exemplary embodiment of the arrangement ofmagnetic material pieces in the enclosure of a wireless power device. Asshown in FIG. 5A, magnetic material pieces are arranged to allow forgreater “capture” of the magnetic flux generated by the wireless powersource. The increase in capture of magnetic flux is related to greatercoupling between the source resonator and device resonator. For example,the arrangement of material pieces as shown in FIGS. 5A-5B may help toincrease coupling by 5% to 10%. The magnetic material pieces in thearrangement shown in FIGS. 5A-5B provide a lower reluctance path formagnetic flux as compared to air. The arrows 502 represent the path oftravel for the magnetic flux from the base magnetic material pieces 424to the overlapping peripheral magnetic material pieces 426. Inembodiments, the overlap 508 between the two planes of magnetic materialmay be approximately 8 mm or greater. In embodiments, the overlap 508may be on the order of the thickness of the ferrite used for theperipheral magnetic material. FIG. 5B shows the gaps 510, 512 betweenthe magnetic material pieces. In embodiments, more than one piece ofmagnetic material may be used to assemble the wireless power device. Inembodiments, the gap 510 may be 0.5 mm, 1.0 mm, or greater. The gap 512between peripheral magnetic material and the base magnetic material mayneed to be also enforced such that it is small. In embodiments, the gap512 between the two planes of magnetic material may need to be 0.5 mm orless. In other embodiments, the gap 512 between the two planes ofmagnetic material may need to 1 mm or less. This gap 512 may have adirect effect on how well the source resonator couples to the deviceresonator. In other words, the device will be able to capture more fluxfrom the source when the gap 512 is small. During the testing of thisgap 512, it was shown that for gaps as great as 2 mm, coupling betweenthe source resonator and the device resonator decreased by 5%-10%. Thisgap 512 may be enforced by using plastic or other non-lossy materialsbetween the two planes of magnetic material.

The gaps 510 between co-planar pieces of magnetic material (such as thatshown in the base portion 424) may also need to be small and uniform toavoid hot spots. This is true for magnetic material in the wirelesspower source and the device. As shown for the wireless power source inFIG. 2B, a continuous piece of magnetic material for the top-hat portionand a continuous piece of magnetic material 208 for the peripheralportion may also be used for the wireless power device.

In exemplary embodiments, the peripheral magnetic material may be madeof ferrite tiles that are 5 mm to 8 mm thick to be able to handle powertransfer levels of 3.3 kW or greater. The base magnetic material may bemade of ferrite tiles that are 2 mm to 5 mm thick. Thinner tiles offerrite in the top-hat as compared to the peripheral tiles may be usedto reduce the overall weight of the packaged device. In some cases,thinner tiles may be sufficient in the base as there is a lessermagnitude of magnetic flux to “capture” in the center of the deviceresonator as compared to the outer edges of the device resonator. Inembodiments, ferrite tiles of less than 5 mm may be too brittle and mayeasily break. It is beneficial to avoid creating breaks in magneticmaterial as hot spots will be created due to the uneven cracks or breakscreated.

FIGS. 6A-6C show top views of exemplary embodiments of a back plate orshield for a wireless power transfer source or a device. In embodiments,a shield may provide mechanical stability to the source or a device. Inembodiments, a shield may provide a thermal path for heat generated bythe operation of the source or device. FIG. 6A shows a shield 602 thatis made from a continuous plate of conductive material, such asaluminum. An advantage to a continuous or unbroken plate is that theeddy currents 604 that are induced due to the source's magnetic field606 will have less resistance to flow. Resistance to eddy currents 604may cause the quality factor of the resonator to decrease. FIG. 6B showsa shield 608 that is made from a plate of conductive material thatdefines at least one hole 610. This provides more resistance to eddycurrents 612 as compared to the continuous plate shown in FIG. 6A.However, a practical advantage of this type of shield may be the abilityto gain access to the inside components of the source without extensivedisassembly. In embodiments, the hole 610 may be covered an additionalsmaller piece of conductive material and act as an “access panel”. Inembodiments, the access panel may provide access to the source or deviceelectronics, such as the impedance matching board of the source ordevice. FIG. 6C shows a shield that is made of four pieces of conductivematerial 614, 616, 618, 620 isolated from one another. These four piecesof conductive material provide more resistance to the induced eddycurrents 622, 624, 626, 628. The shield shown in FIG. 6A has a lesserdetrimental effect on the quality factor of the resonator as compared tothe multiple-piece shield shown in FIG. 6C which has a greaterdetrimental effect on the quality factor of the resonator. Theembodiment shown in FIG. 6C may decrease the quality factor of thesource resonator by approximately 10%, 25%, 50%, or more. For example,the quality factor of the source resonator using the continuous shieldshown in FIG. 6A may be approximately 900 or greater. The quality factorof the source resonator using the multiple-piece shield shown in FIG. 6Cmay be approximately 450 or lower.

FIG. 7A shows a bottom view of the construction of support material 706and magnetic material 704. In embodiments, the support material may beconstructed in four pieces for ease of manufacture. For example, asingle sheet of aluminum may be stamped, folded or otherwise formed tocreate the bracket shape of the support structure 412 shown in FIG. 4.Each of the individual brackets 708 that make up the support materialare approximately 1 cm away from the edge of the magnetic material 704.This spacing 716 may be important in order to not incur additionallosses by being proximate to the source resonator coil 702. Therefore,it may be cost-effective and more feasible to create several linearpieces instead of a single piece with four sides. FIG. 7B shows a sideview of the exemplary embodiment of magnetic material and conductivematerial shown in FIG. 7A. Shown is a side-view of a portion of thesource resonator coil 702, pieces of magnetic material 704, and aportion of conductive material forming a support 706 for the magneticmaterial 704. In embodiments, the source resonator coil 702 may be madeof Litz wire, the magnetic material 704 may be made of ferrite, and theconductive material support 706 may be made of aluminum. Another view ofthe supporting structure 412 is shown in the exemplary source in FIG. 4.Due to the magnetic material 704 being in multiple pieces, magnetic flux708 leaks outside of the surfaces of the magnetic material at thelocations of the gaps between the magnetic material pieces. Theconductive supporting material 706 may be in the form of spaced brackets708 with connections 710 only on the edges. The spaces 712 defined inthe supporting material proximate to the connections 710 providephysical room for the flux leakage 708 to travel without incurringlosses in the support material 706. The connections 710 on the edges ofthe support material and the lower portions of the spaced bracketsprovide a continuous path for eddy currents 714. The connections on theedges 710 of the support material 706 may also provide a continuousthermal path.

FIGS. 8A-8B show magnetic material 802 which is a continuous piece(e.g., a closed square or rectangle) supported by a support material 804which is made of a continuous piece (e.g., a closed square or rectangle)of conductive material. An advantage of a continuous piece of magneticmaterial 802 is that magnetic flux leakage is minimized because thereare no breaks in the magnetic material 802. An advantage of a continuouspiece of conductive material 804 serving as the support piece is thatbreaks in the support are minimized and therefore provide lessresistance to eddy currents.

In exemplary embodiments, it may be critical that the components of thewireless power source and device are held securely in place. Forexample, plastics, thermal interface materials, and other non-lossy ornon-electrically-conductive materials may be used to enforce theinternal structure of the wireless power source and device enclosures.The structure that plastic provides may prevent magnetic material piecesfrom shifting and creating uneven gaps. Magnetic material pieces mayalso be held in place with thermal interface material between a surfaceof the magnetic material pieces and the support structure and betweenthe opposite surface of the magnetic material pieces and conductors ofthe resonator coil. Magnetic material pieces may be further held inplace due to the overall pressure created by the outer packaging of thewireless power source or device, namely the back plate and the topcover. In embodiments, the top cover may be affixed to the back platevia screws made of non-lossy material, such as plastic. In embodiments,the top cover may be affixed to the back plate via clips, snaps, orclasps that are designed into the material of the top cover and/or backplate may be preferred to minimize the number of pieces used in themanufacture of the wireless power source or device. An additionalbenefit may be that the clasps, and the like may be more tamper-proofthan screws.

While this disclosure contains many specific implementation details,these should not be construed as limitations on the scope of thedisclosure, but rather as descriptions of features in connection withembodiments. Features that are described in the context of separateembodiments can also generally be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable sub-combination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can generally be excised from thecombination, and the claimed combination may be directed to asub-combination or variation of a sub-combination. In addition to theembodiments expressly disclosed wherein, other embodiments are withinthe scope of the disclosure.

What is claimed is:
 1. A wireless power device for a vehicle, and forreceiving power from a wireless power source comprising a sourceresonator, the wireless power device comprising: a first plurality ofmagnetic material pieces arranged in a first plane, wherein the firstplurality of magnetic material pieces have a first planar surface and asecond planar surface; a device resonator comprising at least one woundconductor disposed on the first planar surface; and a second pluralityof magnetic material pieces in a second plane, wherein at least one ofthe second plurality of magnetic material pieces overlaps at least oneof the first plurality of magnetic material pieces; wherein a separationbetween the first and second planes is less than 2 mm; and wherein thefirst and second pluralities of magnetic material pieces form alow-reluctance pathway, relative to air, for magnetic flux received bythe device resonator.
 2. The wireless power device of claim 1 whereinthe separation between the first and second planes is less than 0.5 mm.3. The wireless power device of claim 2 wherein a coupling k between thedevice resonator and the source resonator is at least 5% greater thanthe coupling k that would occur between the device resonator and thesource resonator in absence of the second plurality of magnetic materialpieces.
 4. The wireless power device of claim 2 wherein a coupling kbetween the device resonator and the source resonator is at least 10%greater than the coupling k that would occur between the deviceresonator and the source resonator in absence of the second plurality ofmagnetic material pieces.
 5. The wireless power device of claim 1wherein the separation between the first and second planes is uniform.6. The wireless power device of claim 1 wherein the separation ismaintained using a plastic spacer between the first plurality and thesecond plurality of magnetic material pieces.
 7. The wireless powerdevice of claim 1 wherein an overlap between the at least one of thesecond plurality of magnetic material pieces and the at least one of thefirst plurality of magnetic material pieces is equal to or greater than8 mm.
 8. The wireless power device of claim 1 wherein an overlap betweenthe at least one of the second plurality of magnetic material pieces andthe at least one of the first plurality of magnetic material pieces iswithin a factor of 10 of a thickness of the first plurality of magneticmaterial pieces or the second plurality of magnetic material pieces. 9.The wireless power device of claim 5 wherein the first plurality ofmagnetic material pieces each have a thickness between and including 5mm to 8 mm.
 10. The wireless power device of claim 5 wherein the secondplurality of magnetic material pieces each have a thickness between andincluding 2 mm to 5 mm.
 11. The wireless power device of claim 1 whereina coupling k between the device resonator and the source resonator is atleast 5% greater than the coupling k that would occur between the deviceresonator and the source resonator in absence of the second plurality ofmagnetic material pieces.
 12. The wireless power device of claim 1wherein a coupling k between the device resonator and the sourceresonator is at least 10% greater than the coupling k that would occurbetween the device resonator and the source resonator in absence of thesecond plurality of magnetic material pieces.
 13. The wireless powerdevice of claim 1 wherein a gap in the second plurality of pieces near acenter of the at least one wound conductor is 0.5 mm or greater.
 14. Thewireless power device of claim 1 wherein the at least one conductor iswound in the second plane.
 15. The wireless power device of claim 1further comprising a capacitor network coupled to the at least one woundconductor is positioned under the second plurality of magnetic materialpieces.
 16. The wireless power device of claim 15 further comprising aconductor piece positioned between the second plurality of magneticmaterial pieces and the capacitor network.
 17. The wireless power deviceof claim 1 further comprising an aluminum shield positioned adjacent tothe first plurality of magnetic material pieces opposite the at leastone conductor.
 18. The wireless power device of claim 17 wherein thealuminum shield is attached to an underside of a vehicle.